1. Field of the Invention
The present invention relates to the design and placement of particle monitors in a manufacturing environment; and, in particular, relates to the design and placement of particle monitors in a low pressure chamber used in semiconductor processing, such as a plasma etch chamber.
2. Discussion of the Related Art
In semiconductor wafer processing, particle contamination in vacuum processing equipment (called "process tools") is one of the most common sources of yield loss. Consequently, particle monitors for detecting the levels of particles present in a process chamber during processing are developed.
One common technique for monitoring particles in a process chamber places a laser-based particle monitor in the exhaust line of a process tool. In such a laser-based monitor, exhaust gas from the process chamber passes through the particle monitor, carrying in it particles which are detected by laser light scattering. Laser-light scattering occurs in such a particle monitor when a particle passing through the beam scatters light to photocells of the particle monitor. The photocells create an electrical pulse indicating the presence of the particle when the scattered light is received. The use of laser-based sensors and techniques have been described, for example, in: (i) U.S. Pat. No. 4,804,853 to P. Borden et al, entitled "Compact Particle Flux Monitor", Ser. No. 07/041,795, filed on Apr. 23, 1987 and issued on Feb. 14, 1989 as U.S. Pat. No. 4,804,853; (ii) Reissued U.S. Pat. No. Re.33,213, reissued on May 8, 1990, which is based on U.S. Pat. No. 4,739,177 to P. Borden, entitled "Light Scattering Particle Detector for Wafer Processing Equipment", filed on Sep. 16, 1986, and issued on Apr. 19, 1988; (iii) and U.S. Pat. Nos. 5,132,548 and 5,266,798 to P. Borden et al, each entitled "High Sensitivity, Large Detection Area Particle Sensor for Vacuum Applications", having Ser. Nos. 07/582,718 and 07/742,798, issued on Jul. 21, 1992 as U.S. Pat. No. 5,132,548 and Nov. 30, 1993 as U.S. Pat. No. 5,266,798, and filed on Sep. 14, 1990 and Aug. 8, 1991, respectively. In addition, further discussion of laser-based particle monitoring techniques can be found in PG Borden, 10 part series in Microcontamination Magazine, January, February, March, April, May, August, September, October, November, and December issues, 1991.
However, there are some applications in which the use of particle monitors is impractical in the prior art. One such application is found in a low pressure plasma equipment, such as a plasma etcher, in which a turbo pump is used to maintain the reduced pressure required for the plasma etching operation. A turbo pump, which is in principle similar to a propeller, uses a rapidly spinning rotor with numerous blades to create a flow of exhaust gases from the process chamber. Such a pump is described in, for example, High-Vacuum Technology, by Marsbed H. Hablanian, Marcel Dekker, New York (1990), .sctn.7.2, pp. 235-237.
FIG. 1 shows a typical pumping configuration 100 of a plasma etcher for processing one or more semiconductor wafers within its process chamber. As shown in FIG. 1, process chamber 101 includes multiple electrodes (not shown) that are used to maintain a p:Lasma. During operation, a process gas is admitted through numerous fine holes in one of the electrodes, which is typically called the "shower head". Inside process chamber 101, the plasma created by the electrodes and the remaining gas flow uniformly bathe the exposed surfaces of the semiconductor wafers to allow the chemical reactions in a processing step, such as the etching a dielectric film, to occur. The exhaust gases of the chemical reactions are drawn out by a turbo pump 150. A normal mechanical pump (not shown) is connected to turbo pump 150 by pipe 151 to finally draw the gasses to atmospheric pressure.
In configuration 100, a butterfly valve 120 is typically used to maintain the pressure in process chamber 100. Butterfly valve 120 includes a circular plate 102 mounted on a drive shaft 103, which is driven by a motor 104. The entire assembly of butterfly valve 120, including a pump line 105 which houses butterfly valve 120, is also called a weldment. In butterfly valve 120, the variable positions of plate 102 allows a variable restriction of the opening above turbo pump 150. The amount of restriction varies with the angle of plate 102. For example, if the plane of plate 102 is normal to the axis of pump line 105, the restriction is maximum, and the gas flow from the chamber to the turbo pump is substantially entirely blocked. However, if the axis of pump line 105 lies in the plane of plate 102, the gas flow from the process chamber is substantially unblocked. Of course, intermediate angles of plate 102 provide different levels of restriction, Typically, the angle of plate 102 is controlled by a feedback loop, which includes a pressure sensor inside process chamber 101, to provide a stable pressure in process chamber 101. Alternatively, instead of butterfly valve 120, pressure control can also be achieved using a linear gate valve. A linear gate valve consists of a flat plate that slides across the opening of the pump line. Similar to butterfly valve 120, the position of the flat gate in a linear gate valve determines the restriction of the pump line. Thus, further description of a linear gate valve is omitted.
Ideally, particle monitoring should be carried out in the portion of the exhaust gas flow between the process chamber 101 and turbo pump 150, a region referred to as "above the pump". However, particle monitoring is not performed above the pump in the prior art because of the difficulties which are further described below. Instead, particle monitoring is typically performed "after the pump", i.e., downstream from turbo pump 150. For example, in FIG. 1, a particle monitor 152 is shown to be installed in pipe 151. However, after the pump particle monitoring is less desirable because the pumping action of turbo pump 150 provides a large amount of gas mixing. Gas mixing results from gas physically striking either the blades of turbo pump 150 or the boundary layer at the blade surfaces. Consequently, downstream chemical reactions can occur inside turbo pump 150, and particles may be removed or added to the gas flow by the blades and the pump bearings.
However, particle monitoring above the pump is not performed in the prior art for various reasons. First, the throat of turbo pump 150 is typically as close to process chamber 101 as possible, the region between turbo pump 150 and process chamber 100 is bathed with a strong glow from the plasma. The strong glow would swamp out the weaker intensity of any scattered light which is used by a dark field sensor to monitor particles. Second, the gas velocity in the region between process chamber 100 and turbo pump 150 is relatively high because of the low process pressure. In this region, velocities in the range of 10-20 m/sec are common. Particles at this velocity are not efficiently monitored by a dark field sensor, which typically has a maximum sensitivity range at lower particle velocities. Third, the region between turbo pump 150 and process chamber 101 does not provide adequate space for installing a dark field sensor, which includes at least a laser source, large area pick-up lens, and beam stop.